1. Field of the Invention
This invention relates generally to memory systems and more particularly to a system and method for identifying any one surface from among several surfaces which are identically patterned in a stack of memory storage disks.
2. Description of the Background Art
Providing reliable storage and retrieval techniques for digital information is an important consideration of manufacturers, designers and users of computing systems. In magneto-optical (MO) storage devices that use flying heads, digital data is written onto and read from the exposed surfaces on either side of rotating MO disk platters. This digital data is typically written into and read from a series of circular concentric tracks located on the surfaces of the disk platters. In practice, the digital data is read from the exposed surfaces of the disk platters by projecting a laser-generated light beam from a flying MO head onto a selected track while the disk platter is rotating, and then sensing the amplitude and polarization of light reflected back from the surface of the disk platter. Periodically occurring in each track are embedded servo-sector markings. Corresponding servo-sector markings from adjacent tracks are grouped together in the form of narrow radial segments, which allow for precision random-access locating of the data.
Prior to reading data from or writing data to MO disk platters, the platters must be formatted. Formatting refers to the process of writing the servo-sector and track markings onto the platters. This could be accomplished in one of two ways: soft sectoring or hard sectoring. In the soft sectoring process, virgin platters are assembled into a complete disk assembly. Then the servo-sector and track markings are written to the surfaces of the platters using the flying MO heads of the disk assembly. Soft sectoring has certain technical advantages, but has one great cost disadvantage. It requires large amounts of time, often many hours, during the manufacturing process to perform the soft-sector formatting. To stay economically competitive, most MO disk drive manufacturers have therefore opted for hard sectoring.
In the hard-sector formatting process, the servo sector and track markings are stamped into the raw MO disk platters during the platter manufacturing process. In a typical process, a pair of polycarbonate plastic blanks are each coated on one side, the media side, with a thin magnetic film, and then each is stamped in a female mold. These two disk halves are then glued together at the sides opposite the media sides, the glue sides. It takes only a few seconds to stamp and glue together an MO disk platter in this manner, and therefore this method yields great cost savings when compared with soft sector formatting. Additional cost savings accrue from formatting all of the platters identically. These cost savings come from the tooling cost savings from having only one pair of molds (for top halves and bottom halves of the platters) and the inventory cost savings from only needing to stock one kind of platter. It is thus advantageous to use all identical hard-sectored platters. However, there are certain technical problems encountered when using hard-sector formatting.
One technical problem associated with identical hard-sector formatting in a multi-platter MO disk is that there is no technical method available to validate which platter's surface is being accessed simply by reading the servo-sector and track markings. Currently, in a multi-platter MO disk, the drive electronics select a particular flying MO head via a head switch, and makes the assumption that there is a one-to-one correspondence between selected flying heads and MO disk platter surfaces. This assumption is false in the event that the head switch is not perfectly reliable. If, for example, the head switch was commanded to connect the drive electronics to the flying MO head corresponding to the top surface of MO disk platter number 1, and the head switch mistakenly connected the drive electronics to the flying MO head corresponding to the top surface of MO disk platter number 2, it would likely follow that data intended to be written on the top surface of MO disk platter number 1 would instead be mistakenly written on the top surface of MO disk platter number 2. This inability to independently validate the operation of the flying head switch thus may lead to data corruption in the MO disk.
A second problem concerns the radial position alignment of the tracks. The geometric center of the hole punched in the middle of the disk halves is not necessarily the same as the geometric center of the hard-sector tracks. When the top and bottom halves of the disk platter are glued together, the geometric centers of the tracks on the top half and on the bottom half will not be aligned. This causes head tracking problems which may only be overcome at the cost of more complicated servo drive electronics.
This second problem continues when the assembled disk platters are mounted on a common rotational spindle to form a multi-platter disk drive. Not only are the tracks on the top and bottom halves of each platter not in radial alignment with each other, but also the tracks on the various platters are not in radial alignment with respect to the common rotational axis of the spindle. This again causes head tracking problems which may only be overcome by servo drive electronics.
Referring now to FIG. 1A, an exploded view of the assembly of a double-sided MO disk platter 110 is shown. Looking at the media side 112 of the top half 114, if the top half 114 rotates in a counter-clockwise direction, the attached bottom half 116 will rotate (in reference to its own media side 118) in a clockwise direction. Because the data in the servo sectors 120 must be read in sequential order as the data bits pass the heads, the data in the servo sectors 120 needs to be written in different directions on the top half 114 and bottom half 116. Therefore the top half 114 and the bottom half 116 each require their own mold. In FIG. 1A the top half 114 and the bottom half 116 have previously been pressed, respectively, in a top half mold and a bottom half mold (not shown). They are then glued together to form a double-sided disk platter 110 as shown in FIG. 1A with the hard-sector formatted surfaces exposed on the outside.
Conventionally the top half 114 and the bottom half 116 are glued together in random rotational relationship. They are also glued together in poor radial alignment. This is because there exist no alignment markings which can be interpreted with an assembly-line quality microscope or which can be selected with alignment tooling. There are flying MO head readable markings on the hard-sectored platters' surfaces, but these markings take the form of embossed pits or polarization-shifted patches of approximately 0.7 microns across. It would be necessary to read several of these patches in sequence to determine which servo sector was being examined. This is not practicable using current assembly line equipment or techniques.
Referring now to FIG. 1B, a pair of conventional disk platters 110 for a two disk platter MO disk drive is shown. These disk platters 110 are mounted for rotation on a spindle 122. The disk platters 110 are assembled in random rotational alignment and in poor radial alignment for the same reason that the two halves 114, 116, of each platter are assembled in random rotational alignment and in poor radial alignment: there exist no markings on the platters which could be used by an assembler to mount the platters on the spindle with a known rotational alignment or to align the geometric centers of the tracks. Because the platters are assembled in random rotational alignment, there exists no indication which would independently show that the flying MO head switch correctly switched the heads. For example, consider switching from the head on the upper surface 128 of disk 1 124 to the head on the lower surface 134 of disk 2 126. An instant before the switch was commanded to change heads, the head on the upper surface 128 of disk 1 124 was over a known servo sector. Once the switch changes heads, data from a new surface will be read. The drive electronics may be able to determine if the head which has just been connected is over an upper surface or a lower surface because the upper surfaces and lower surfaces, stamped in different molds, may contain different format information. However, because all the bottom halves of the disk platters are identical, there is no independent way to know if the switch correctly changed to the head over the lower surface 134 of disk 2 126 or incorrectly changed to the head over the lower surface 130 of disk 1 124.
Similarly, because the platters are assembled on the spindle without regard for radial alignment, the tracks are in poor radial alignment with respect to the rotational axis of the spindle.
Therefore there exists a need for a system and a method for assembling the top halves and bottom halves of the disk platters, and the disk platters in a multi-platter MO disk drive, in a known rotational alignment and in good radial alignment of the servo sectors and tracks.